The operating frequency of high-frequency electronic circuits is increasing more and more, so it is necessary to accurately measure electric characteristics of an electronic device used in such a circuit in a high frequency range. Among items to be measured at high frequencies, a reflection coefficient and a transmission coefficient are important. A reflection coefficient is the ratio of a wave reflected from a DUT to an incident wave applied to the DUT. A transmission coefficient is the ratio of a wave transmitted through a DUT to an incident wave applied to the DUT. They are all vector values (or complex quantities). However, in terms of actual use, only amplitude information (scalar values) for each is important in most cases.
When a wave reflected from or transmitted through a DUT is observed by a measurement system, ordinarily, it has many errors of the measurement system. By removal of the errors of the measurement system from an observed amount, true characteristics of a DUT can be obtained. To do this, it is necessary to observe the reflected wave or transmitted wave as a vector value (to vector-measure). If only amplitude information (scalar value) for them is observed, it is difficult to accurately remove errors of the measurement system. In other words, although only the amplitude for a reflection coefficient or transmission coefficient is important in terms of actual use, a vector value is required to accurately measure them.
A vector network analyzer (VNA) is typically used in vector measurement of high-frequency characteristics in the extremely high frequency or higher range. To determine a matrix of scattering coefficients (e.g., a reflection coefficient and a transmission coefficient), a VNA applies a measurement signal to a DUT and measures the amplitude ratio of each of a reflected wave and a transmitted wave to the measurement signal and the phase difference between the measurement signal and each of the reflected wave and the transmitted wave. That is, a VNA is a measuring instrument in which a signal source and a vector detector are combined. A known VNA employs a heterodyne detecting scheme using, as an important part of a structure of the vector detector, a phase locked loop (PLL) that includes a local oscillator and a mixer.
However, for the structure of the VNA, there is a problem of a considerable increase in cost caused by an increase in the number of stages of the local oscillator and mixer of the PLL with a rise of a measured frequency or by other reasons. In addition, an increase in frequency leads to a loss occurring in frequency conversion and a decrease in the purity of a measurement signal, thus resulting in a problem in which a high-precision measurement is difficult.
Non-Patent Document 1 discusses a process for measuring electric power with a measurement system that performs four measurements on an incident wave and reflected wave and deriving a phase difference from the four measurement values of electric power using a system parameter of the measurement system. For this process, the phase difference, which becomes more difficult to be measured as the frequency increases, is derived based on measurement of a scalar value being an electric-power value being a fundamental measured volume in measurement of electromagnetic waves, the measurement accuracy being virtually independent of frequency. This can remove the above-described drawbacks in a VNA. However, it is necessary to measure four electric powers by four electric-power measuring instruments to measure reflection, so there is a problem in which the overall size of a measuring instrument is increased and the cost is also increased.
Patent Document 1 describes a process for improving the measurement accuracy by increasing the number of electric-power measurement values from four to five and making it possible to compare the amplitude ratios between an incident wave and a reflected wave using a basic measurement principle similar to that in Non-Patent Document 1. In this process, five electric powers are measured by five electric-power measuring instruments to measure reflection. This causes a further increase in the overall size of a measuring instrument, and also raises the cost.    Non-Patent Document 1: G. F. Engen, “The six-port reflectometer: An alternative network analyzer, “IEEE Transactions on Microwave Theory and Techniques, vol. MTT-25, no. 12, pp. 1075-1080, December 1977    Patent Document 1: Japanese Patent No. 3540797